1. Field of the Invention
The invention relates to a method and apparatus for using an electromagnetic technique to monitor physiological changes in the brain. More particularly, the invention uses an electromagnetic field to non-invasively measure impedance changes at a localized point within an animal or human brain over time. For example, these localized impedance measurements can be used to detect and monitor the advent and growth of edematous tissue, or the process of hydrocephalus.
2. Description of the Prior art
It is important in diagnosing and treating various life-threatening conditions, such as brain edema and hydrocephalus, to monitor the time-trends of physiological changes in the brain. Brain edema, which is an increase in brain volume caused by grey and/or white brain tissue absorbing edematous fluid, can develop from general hypoxia; from cerebral hemorrhage, thrombosis, or embolus; from trauma (including post-surgical); from a tumor; or from inflammatory diseases of the brain. Brain edema can directly compromise vital functions, distort adjacent structures, or interfere with perfusion. It can produce injury indirectly by increasing intracranial pressure. In short, brain edema is often a life-threatening manifestation of a number of disease processes.
There are several effective therapeutic measures to treat brain edema. These include osmotic agents, corticosteroids, hyperventilation to produce hypocapnia, and surgical decompression. As with all potent therapy, it is important to have a continuous measure of its effect on the manifestation, in this case, the brain edema.
All current techniques for measuring physiological changes in the brain, including the manifestation of brain edema, have shortcomings in providing continuous or time-trend measurements. Intracranial pressure can be monitored continuously, but this is an invasive procedure. Furthermore, intracranial compliance is such that substantial edema must occur before there is any significant elevation in pressure. When the cranium is disrupted surgically or by trauma, or is compliant (as in infants), the pressure rise may be further delayed. These patients are often comatose, and localizing neurological signs are a late manifestation of edema. Impairment of respiration and circulation are grave late signs. Thus, clinical examination is not a sensitive indicator of the extent of edema. X-ray computed tomography (CT) scanning can produce valuable evidence of structural shifts produced by brain edema, and it is a non-invasive procedure. Structural shirts, however, may not correlate well with dysfunction, especially with diffuse edema. Furthermore, frequent repetition is not feasible, particularly with acutely ill patients. NMR proton imaging can reveal changes in brain water, it does not involve ionizing radiation, and it is non-invasive. However, it does not lend itself to frequent repetition in the acutely ill patient. PET scanning can reveal the metabolic disturbances associated with edema and will be invaluable in correlating edema with its metabolic consequences. However, it too is not suited to frequent repetition.
For these reasons it would be a significant advance to have a measurement which (1) gives reliable time-trend information continuously; (2) is non-invasive; (3) does not depend upon the appearance of increased intracranial pressure, and (4) can be performed at the bedside even in the presence of life-support systems.
As will be discussesd in detail subsequently in this application, Applicant has related localized impedance changes in the brain with physiological changes in the brain. Applicant was the first to identify that edematous tissue has a significantly different conductivity from healthy white or grey matter.
To non-invasively detect such an impedance change, Applicant has invented a method and apparatus which uses an electromagnetic field for sensing such an impedance change at localized portions of the brain. U.S. Pat. No. 3,735,245 entitled "Method and Apparatus for Measuring Fat Content in Animal Tissue Either in Vivo or in Slaughtered and Prepared Form", invented by Wesley H. Harker, teaches that the fat content in meat can be determined by measuring the impedance difference between fat and meat tissue. The Harker apparatus determines gross impedance change and does not provide adequate spatial resolution for the present use. As will be discussed in detail later, brain impedance measurements must be spatially localized to provide a useful measure of physiological changes. A general measurement of intracranial conductivity would not be revealing, since as in the case of brain edema, the edematous fluid would initially displace CSF fluid and blood from the cranium; and, since these fluids have similar conductivities, a condition of brain edema would be masked.
U.S. Pat. No. 4,240,445 invented by Iskander et al teaches the use of an electromagnetic field responsive to the dielectric impedance of water to detect the presence of water in a patient's lung. The Iskander et al apparatus generates an electromagnetic wave using a microwave strip line. Impedance changes within the skin depth of the signal will cause a mode change in the propagating wave which is detected by associated apparatus. Therefore, Iskander et al uses a different technique from the present invention and does not detect conductivity variations with the degree of localization required in the present invention. U.S. Pat. No. 3,789,834, invented by Duroux, relates to the measurement of body impedance by using a transmitter and receiver and computing transmitted wave impedance from a propagating electromagnetic field. The Duroux apparatus measures passive impedance along the path of the propagating wave, whereas the present invention measures localized impedance changes in brain matter and fluid by measuring the eddy currents generated in localized portions of the brain matter and fluid. None of the above-cited references contemplate measuring localized impedance changes in the brain to evaluate physiological changes in the brain, such as the occurrence of edematous tissue, and none of the references teach an apparatus capable of such spatially localized impedance measurements.